The invention relates generally to detecting core faults and more particularly to testing laminated cores of electric machines for interlamination short circuits.
Laminated stator cores are tested for interlamination shorts during manufacture and in operation in the course of maintenance operations. Core faults, such as faults caused by short circuited laminations, in large electric machines can be highly destructive.
As described in Posedel, U.S. Pat. No. 4,996,486, issued Feb. 26, 1991, one inspection technique includes ring excitation of the stator lamination with rated induction. This method, which indicates the effect of currents due to interlamination shorts by local temperature differences (which can be detected by an infrared scanner, for example), requires a high-power and controllable high-voltage source and excitation windings with large cross sections. The high power requirements cause this method to be impractical for field applications. Furthermore, this method includes specially winding the stator core.
As further discussed in aforementioned U.S. Pat. No. 4,996,486, another inspection technique involves measuring current fields due to interlamination shorts with weak induction. Only a low-voltage supply connection is required to magnetize the core stack at a lower magnetic flux level as compared to the rated level. This technique is commonly referred to as an ELectromagnetic Core Imperfection Detector (EL CID) test. In this method, the flux along each tooth is scanned with a special detector coil system to look for anomalies. Again, this method includes specially winding the stator core.
It would be desirable to have a method that does not require specially winding the stator core.
Briefly, in accordance with one embodiment of the present invention, a method for detecting core faults comprises (a) positioning a magnetic yoke near at least one tooth of the core, the magnetic yoke being wound by a winding; (b) supplying current to the winding to inject magnetic flux into the at least one tooth of the core; (c) measuring a signal (meaning at least one signal) resulting from the injected magnetic flux; and (d) using the measured signal to detect core faults.
In accordance with another embodiment of the present invention, a method for using a laminated magnetic yoke to measure a plurality of sections of a magnetic core comprises: (a) providing a compliant plunger for supporting yoke laminations of the laminated magnetic yoke; (b) positioning the magnetic yoke near a section of the core; (c) lowering the magnetic yoke until at least some of the yoke laminations are in contact with core laminations of the magnetic core; (d) taking a measurement; (e) lifting the magnetic yoke at least until none of the yoke laminations is in contact with any of the core laminations; and (f) repeating (b)-(e) near at least one different section of the core.